Gas turbine control

ABSTRACT

A control system for a gas-coupled gas turbine engine particularly directed to control of the setting angle of the power turbine nozzle of the engine and to correlation of the power turbine nozzle setting with fuel supply and with operating parameters of the engine. The control provides for operation of the engine from closed to full open throttle, with closed throttle calling for idle speed of the engine. The initial stage of opening the throttle develops an increasing nozzle reference temperature signal which is matched with a turbine temperature signal to control a servo which varies the setting, and therefore area, of the turbine nozzle. After the initial stage of throttle opening, the reference temperature signal increases with engine speed, which increases in response to an increasing throttle signal. The actual level of the nozzle reference temperature is also a function of engine inlet temperature up to a given level of this temperature. Throttle position generates a speed reference signal which is matched with actual gas generator speed to control fuel supply to the engine. The speed reference signal is coordinated with the nozzle reference temperature signal so that there is a smooth transition from the increasing nozzle reference temperature over the initial range of throttle movement to the increasing engine speed reference signal over the remainder of the travel of the throttle up to or near 100% rated power output signal. 
     The control also responds to acceleration of the gas generator to vary the nozzle reference signals turbine inlet temperature with an acceleration reference temperature signal responsive to engine inlet air temperature, which signal is used to limit engine fuel flow during acceleration. Upon occurrence of an engine acceleration above idle, the opening of the turbine nozzle is limited to improve energy division between the gas generator and power turbines. In one embodiment this limitation is maintained during the duration of acceleration; in another the limitation of nozzle opening is progressively relaxed during the acceleration. 
     The nozzle opening limitation is overridden during starting of the engine and acceleration to idle, specifically by an override responsive to operation of the starter to crank the gas generator.

Our invention relates to controls for gas turbine engines, andparticularly to an integrated control for an engine of the gas-coupledtype suitable for driving a motor vehicle or similar service. Theinvention is primarily directed to improved means for varying thesetting of turbine nozzle vanes of the power turbine of the engine so asto optimize the operating characteristics of the engine. Also, it isdirected to interconnections between the means which generate a signalcontrolling gas generator speed and the nozzle control signal so as toachieve an increasing engine temperature at constant gas generator speedby closing the nozzle during initial movement of the engine powercontrol or throttle, and thereafter to increase gas generator speed asthe power control is further advanced.

Objects of our invention are to improve the control and operatingcharacteristics of gas turbine engines, particularly of engines intendedfor vehicle service; to provide improved turbine nozzle angle settingcontrols for a gas turbine engine; to provide improved coordinated fueland turbine nozzle controls for a gas turbine; and to provide superiorelectronic controls for a gas turbine engine.

The nature of our invention and its advantages will be clear to thoseskilled in the art from the succeeding detailed description of thepreferred embodiment of the invention and the accompanying drawingsthereof.

FIG. 1 is a schematic diagram of a gas turbine engine control system.

FIG. 2 is a diagram of electrical circuits of the control system.

FIG. 3 is a partial diagram illustrating a modification of the circuitof FIG. 2.

FIG. 4 is a plot illustrating the nature of an acceleration referencetemperature signal.

FIG. 5 is a plot illustrating variations of a nozzle referencetemperature signal with gas generator speed and engine inlettemperature.

FIG. 6 is a plot illustrating variations of the nozzle referencetemperature signal and a gas generator speed reference signal withvariations in throttle signal.

FIG. 7 is a plot illustrating variations of the nozzle referencetemperature signal with time during gas generator acceleration withvarious modes of control.

GENERAL DESCRIPTION

FIG. 1 provides a basis for a general explanation of the nature of ourpreferred controls and for the succeeding detailed description ofpreferred circuits. FIG. 1 illustrates a gas turbine engine 2 which maybe of conventional type including a compressor 3, combustion apparatus4, and a turbine 6. The turbine is connected to drive the compressor,which supplies compressed air to the combustion apparatus where fuel isburned to heat the air, and the resulting combustion products drive theturbine. This part of the engine is called a gas generator. The gasgenerator supplies motive fluid to a power turbine 7 which drives apower output shaft 8. The power turbine includes a variable nozzle 10which may be of known type in which the direction of flow of the motivefluid to the power turbine is altered by varying the setting angle ofthe vanes. Varying the setting angle also varies the area of the nozzle,and thus the division of energy between the turbines 6 and 7. Suchnozzle structures are well known, and the nozzle structure may be of anysuitable known type. As illustrated, the nozzle includes a unison ring11 coupled to the vanes which is rotated about the axis of the engine bya servomechanism 12 which reciprocates a rod 14 coupled to the ring 11.

The engine will ordinarily include a regenerator (not illustrated) toheat air flowing from the compressor to the combustion apparatus by heatexchange with the exhaust from turbine 7. This is conventional.

Our invention is largely concerned with the control of theservomechanism 12. It is also concerned with the relation of suchcontrol to the supply of fuel to the combustion apparatus of the engine.As illustrated, the fuel is supplied to the combustion apparatus from afuel regulator 15 through a fuel line 16. The fuel regulator may be anysuitable apparatus which responds to a control signal to determine theamount of fuel supplied. It may be a variable displacement pump drivenby the engine or otherwise. It may be means for throttling fuel flowfrom a pump to the engine and by-passing excess fuel back to the pumpinlet. There is no need to describe this in detail. One fuel regulatorwhich might be employed is described in U.S. Pat. No. 3,732,039 ofCarothers issued May 8, 1973.

Power output level of the engine is set, and acceleration anddeceleration are controlled, by a power level control 18 embodiedspecifically in the common foot throttle or accelerator pedal as isordinarily employed in motor vehicles. The foot throttle transmits asignal, represented by the character theta, indicative of the degree ofmovement of the accelerator pedal. This movement is translated bysuitable means into a signal adapted to be employed in the electricalcontrol system. Preferably, this is accomplished simply by apotentiometer 19 energized from a constant voltage source of 10 volts.The slider which is coupled to the foot throttle transmits an electricalpotential which represents the power demand input setting, which may bereferred to as the power level control signal or power signal.

As indicated, this power signal is transmitted over a line 20 to a speedreference signal generating circuit 22 and ultimately through a line 23to a fuel control circuit 24. The fuel control circuit, in response tothis input and other inputs to be described, provides a signalrepresenting required fuel flow on a line 25 to the fuel regulator 15and to a turbine inlet temperature compensating circuit 26.

Our preferred system responds to three significant parameters of engineoperation: namely, engine inlet temperature represented by T₁, gasgenerator speed represented by N₁, and turbine inlet temperaturerepresented by T₄. The transmitters or transducers for these quantitiesare represented respectively by 27, 28, and 30. Each of these devicesprovides an electrical output which is amplified in amplifiers 31, 32,and 33 respectively. The inlet temperature signal from amplifier 31 istransmitted through a line 34 to the speed reference signal generator 22where it coacts with the power signal from line 20 in determining themagnitude of the speed reference signal through line 23 to the fuelcontrol 24.

Line 34 also transmits engine inlet temperature to an accelerationreference temperature signal generating circuit 36. The accelerationreference signal is generated in response to the signal of inlettemperature and a signal of gas generator speed transmitted through aline 38 from the amplifier 32. The structure of the accelerationreference signal generator is immaterial to the present invention. Itgenerates an output signal as illustrated in the plot of FIG. 4. This isa modified inlet temperature signal, as will be further explained.

The acceleration reference temperature signal is transmitted through aline 39 to the fuel control where this is compared with a signalrepresenting engine temperature as a means to limit fuel duringacceleration. The fuel control also receives an input of gas generatorspeed from line 38 and an input of compensated turbine inlet temperaturefrom the compensator 26 through a line 40.

The compensated turbine inlet temperature signal is derived from theturbine inlet temperature as measured by thermocouples or othertransducers 30 and amplified in the amplifier 33. This is transmitted tothe compensator through a T₄ line 41. The measured T₄ signal is modifiedso as to be more nearly representative of instantaneous turbine inlettemperature during transients, and further modified to improve theacceleration characteristics of the gas generator. For this reason, theturbine inlet temperature compensating circuit 26 receives inputs of gasgenerator speed and fuel demand signal in addition to the uncorrectedtemperature signal. The output of the compensator through line 40 iscompared in the fuel control with the acceleration reference temperaturelimiting signal supplied through line 39 to reduce fuel as necessary toprevent overtemperature during acceleration. In steady state operation,fuel normally is controlled by comparison of gas generator speed withthe speed reference signal. The fuel control may receive other inputssuch, for example, as inputs of power turbine speed or acceleration fromoverride logic circuits 42 to reduce fuel or shut down the engine incase of threatened power turbine runaway, but such matters areimmaterial to the present invention and therefore are not explainedhere.

At this point, we have described the engine control system except forthat portion particularly related to control of the variable nozzle 10.As a matter of general introduction to this subject, a nozzle referencetemperature generating circuit 43 receives an input of engine inlettemperature through line 34, an input of gas generator speed throughline 38, an input of acceleration reference temperature through line 39,and an input from the power level control through line 20. The circuit43 transmits through line 46 a temperature limit signal to a nozzlecontrol circuit 47. The variable turbine nozzle is controlled duringsteady state engine operation by circuit 47, which receives also aninput of compensated turbine inlet temperature from line 40. Turbinenozzle opening is limited by a nozzle limit circuit which receives aninput representative of turbine nozzle vane angle or setting, beta,through a line 49. The setting signal is taken from a potentiometer 50,energized from the 10 volt circuit, by a slider moved by the rod 14 ofthe nozzle servo 12, or by any other suitable connection to the turbinenozzle vanes. The potential on line 49 represents the turbine nozzlevane angle and nozzle area.

For improved acceleration of the engine and of the vehicle operated byit, there is also provided a nozzle limit logic circuit 51 whichreceives an input of compensated turbine inlet temperature from line 40and an input of acceleration reference temperature signal from line 39.The limit control delivers a correcting signal through a line 52 to thenozzle limit circuit 48. The limit circuit compares the feedback signalon line 49 with the limit signal on line 52, and transmits an overridingsignal to the nozzle control circuit through line 53 to limit nozzlearea during engine acceleration.

To modify the operation of the nozzle during starting of the engine, thelimit logic circuit 51 also is connected through a line 55 to theenergizing circuit of a starter 54 which is connected to the gasgenerator 2 to crank it for starting the engine.

Passing over for the time being the preferred structure and operation ofcircuits 22, 36, 43, 47, 48, and 51, we may point out that the output ofcontrol circuit 47 is transmitted as a control signal through a line 56to a power amplifier 58 the output of which controls the operation ofthe servo 12. As illustrated, the power amplifier output is connectedthrough a line 59 and a coil 60 to ground. The coil 60 operates an input62 to the servomechanism 12. The nature of the servomechanism isimmaterial but, for example, it might be a hydraulic servomechanism thedirection and extent of movement of which is controlled by movement of,or force exerted on, the input 62.

While it is immaterial to our present invention, it is ordinarilydesirable to provide means to brake the power turbine by increasing theangle of the nozzle 10 much beyond the normal operating range so as toreverse the swirl of motive fluid entering the power turbine. Asindicated in the schematic diagram, a brake control 63 is connected tothe power amplifier. Such a brake control may override the input throughline 56 to drive the servo to full travel in the direction to reversethe turbine vanes. It is well known such action may be desired andeffected in response to overspeed or unduly high acceleration of thepower turbine; or in response to operation of the power turbine above arather low speed when the throttle 18 is calling for idle engineoperation, to provide a measure of braking of the vehicle duringcoastdown.

Steady-State Nozzle Control

We may now proceed to a description of the preferred electrical circuitsof the control, and with this a further explanation of the operation ofthe control. In connection with the description of the circuits,component types and values are identified for completeness of disclosureand to facilitate practice of the invention. However, it is to beunderstood that these are subject to variation in response to changingparameters of the engine and developments in the electronic arts, andsimply as a matter of choice. The identification of component types andcharacteristics is therefore not to be considered in any sense aslimiting the invention. All operational amplifiers are type S5558V madeby Signetics Corporation, and diodes are type DS-31.

Referring to FIG. 2, components and connecting lines identified on FIG.1 bear the same numbers on FIG. 2.

The nozzle reference temperature generating circuit 43 has an input fromthe acceleration reference temperature generating circuit 36. The signalline 39 is connected to the plus input of an operational amplifier 66.The output terminal of amplifier 66 is connected through a resistor 67(10 kilohm) to the minus input, which also is connected to the midpointof a voltage divider. This voltage divider, energized from the plus 10volt supply to ground, comprises a 33 kilohm resistor 68 and a 51 kilohmresistor 70. Operational amplifier 66 may be regarded as a follower orscaler, the gain and the DC shift of the amplifier being determined bythe resistors 67, 68, and 70.

Referring to FIG. 4 for the nature of the acceleration reference signal,it will be seen that the acceleration reference temperature signal is alinear function of engine inlet temperature up to 2000°F. at 60°F. inlettemperature, above which there is no increase in the accelerationreference temperature. Below 50% gas generator speed, which is idle, theacceleration reference temperature decreases with speed along the line71 of the plot down to about 30% speed, below which the accelerationreference temperature is limited at 1650°. It is a feature of ourpresent invention that this signal is employed for control of theturbine nozzle.

The amplified acceleration reference signal from operational amplifier66 is mixed with a signal of gas generator speed, N₁, from amplifier 32,the output of which is connected through an 11 kilohm resistor 72 to ajunction 74. The output terminal of amplifier 66 is connected to thisjunction through a 5.1 kilohm resistor 75. Junction 74 is connected tothe plus input terminal at 77 of a second operational amplifier 76through a 10 kilohm resistor 78. Resistors 72 and 75 determine therelative effect on operational amplifier 76 of the accelerationreference and gas generator speed signals. The input resistor 78provides for limiting the potential level on the plus input of amplifier76 by operational amplifiers 79 and 80, as will be explained.

Junction 74 is also connected through a 20 kilohm resistor 82 to theslider of a 5 kilohm potentiometer 83 connected between the regulated 10volt supply and ground. Potentiometer 83 provides a means forcalibrating the output of amplifier 76.

Operational amplifier 76 provides the nozzle reference signal on line 46connected to the output terminal of the amplifier. The direct feedbackfrom the output to the minus input causes operational amplifier 76 tooperate as a voltage follower for isolation of line 46 from the input onthe plus terminal of the amplifier. The circuits so far described,including operational amplifiers 66 and 76, provide a signal on line 46which increases with the acceleration reference signal (which isproportional to inlet temperature up to 60°F.) and also increases withgas generator rpm. This characteristic is illustrated in the slopingcharacteristic lines on FIG. 5 which will be discussed below.

Operational amplifier 79 acts as a limiter responsive only to theacceleration reference signal which limits increase of the nozzlereference signal, preventing the signal from increasing above the 100%speed value if the engine should go overspeed for any reason or if thecalibration of the operational amplifier 76 is erroneous. Theacceleration reference signal on line 39 is supplied to the plus inputof amplifier 79 through a voltage divider consisting of resistor 84 (2kilohm) and resistor 86 (51 kilohm). The output terminal of amplifier 79is connected through a diode 87 to the minus input of amplifier 79 andthe plus input 77 of amplifier 76. Diode 87 prevents amplifier 79 fromincreasing the plus input to amplifier 76, but it can hold the inputdown in the event the potential level at junction 74 is higher than thatsupplied to the input of amplifier 79 through the voltage divider 84,86. This voltage divider slightly reduces the potential at the input ofamplifier 79 below that supplied to amplifier 66.

The nozzle reference temperature signal is also modified under certainconditions by a portion of the circuit which responds to engine inlettemperature and throttle position. The T₁ signal from amplifier 31 online 34 is supplied through 5.1 kilohm resistor 88 to a junction 90which is connected to the minus input of operational amplifier 80.Junction 90 also receives the nozzle reference signal from line 46through a 7.5 kilohm resistor 91. Junction 90 is connected through a 30kilohm resistor 92 to the minus input of an operational amplifier 94forming part of the speed reference circuit 22. The power control inputpotentiometer 19 is connected between plus 10 volts and engine inlettemperature signal line 34. The movable contact, which is operated bythe foot pedal, produces a signal on line 20 which is connected directlyto the plus inputs of operational amplifiers 80 and 94. The outputterminal of operational amplifier 80 is connected to the plus inputpoint 77 of operational amplifier 76 through a diode 95 so thatoperational amplifer 80 may reduce the plus input of operationalamplifier 76, but does not increase it.

Assuming that there is no effect due to current through resistor 92, thenormal state of affairs at low throttle settings, it follows that thepotential on the minus input of operational amplifier 80 (junction 90)equals the sum of T₁ times a first constant and nozzle reference signaltimes a second constant, the constants being dependent upon the relativevalues of resistors 88 and 91. As inlet temperature or the nozzlereference signal increases, the minus input of operational amplifier 80increases. The input to the plus terminal of operational amplifier 80 isa potential equal to the inlet temperature signal potential plus thetatimes (10 volts minus the temperature signal). It is thus proportionalto the relative movement of the power control from the idle positiontoward full throttle. The idle setting may be fixed by a limit on returnmovement of the foot pedal. Whenever the minus input to amplifier 80becomes higher than the plus input, amplifier 80 will conduct to reducethe plus input potential of amplifier 76 and thereby the nozzlereference signal, until the nozzle reference signal is reducedsufficiently to equalize the inputs to operational amplifier 80. Thus,at closed throttle, the nozzle reference signal is controlled by inlettemperature, which limits it to a value below that which would be due tothe acceleration reference and speed inputs.

Referring to FIGS. 5 and 6, from approximately 0 to 10% throttle, thenozzle reference temperature signal increases as indicated by verticalline 96 on FIG. 5 and line 97 on FIG. 6. The indicated points on line 96of FIG. 5 represent closed throttle nozzle reference temperatures fromabout 850° to about 1350° at varying inlet temperatures at the 50% speedcondition of the engine. The engine speed is maintained constant, asindicated by line 98 on FIG. 6, but fuel is increased because the nozzleopening is decreased. Thus, less of the total fuel energy is availableto drive the compressor and more is available to drive the powerturbine. The 10% throttle position might provide sufficient power todrive a vehicle at a constant speed of about 30 mph. Above this point,as the power control input increases, the engine gas generator speedincreases along a line such as 99 in FIG. 6. This is accompanied by anincrease in nozzle reference temperature due to the increase in speedalong the family of lines 100 in FIG. 5. The lower lines represent T₁values below 60°F. The increasing temperature and increasing speed ofthe gas generator result in a greater amount of energy available to thepower turbine for vehicle propulsion or other purposes. The broken line103 in FIG. 6 represents in a general way the variation of nozzlereference temperature above 10% throttle, which is not directlydependent upon throttle, but rather upon speed and accelerationreference temperature, for 60°F. inlet temperature.

Operational amplifier 80 acts as a discriminating device controlling theswitch-over from constant idle speed with variable nozzle to variablespeed operation at higher throttle settings.

The minimum amount of the nozzle reference signal is determined by T₁when the potential increment from throttle potentiometer 19 is zero. Asthe throttle input increases, the output of operational amplifier 80increases, thereby pulling the plus input to operational amplifier 76down less and less with increasing throttle until the point is reachedat which the potential at the output of operational amplifier 80 equalsthe potential at junction 74 less the diode drop, at which point the gasgenerator speed and acceleration reference signals take over control anddetermine the nozzle reference signal on line 46, as indicated by thelines 100 on FIG. 5.

The horizontal line segments 104 on FIG. 5 represent the maximum limiton the nozzle reference signal at the normal 100% throttle value imposedby the acceleration reference signal transmitted through amplifier 79and diode 87 to the plus input of amplifier 76 if potential at junction74 rises above this value.

Before proceeding with the nozzle angle control system includingcircuits 47, 48, and 51, it may be best to proceed with the descriptionof the speed reference circuit 22. The output terminal of operationalamplifier 94 is connected to the base of an NPN transistor 106, DelcoService type 67, the emitter of which is connected directly to the minusinput of the amplifier. The collector of transistor 106 is energizedfrom the plus 10 volt regulated source through a 10 kilohm resistor 107and a diode 108. The collector of transistor 106 is also connected tothe base of a PNP transistor 110, Delco Service type 83. The emitter oftransistor 110 is energized from the plus 10 volt regulated supplythrough a 270 ohm resistor 111, and its collector is connected through 1kilohm resistor 112 to the plus input of an operational amplifier 114.This operational amplifier acts as a follower and provides the speedreference signal to the fuel control 24 through line 23 which is fedback to the minus input of the operational amplifier.

The plus input of operational amplifier 114 is also connected to anoperational amplifier 115 in a maximum limiting circuit and anoperational amplifier 116 in a minimum setting circuit. The plus inputof operational amplifier 115 is energized off a variable settingpotentiometer 118 energized between the 10 volt control DC unit andground and the plus input of operational amplifier 116 is similarlyenergized through a potentiometer 119. The settings of thepotentiometers determine the maximum and minimum limit values of thespeed reference signal. The output terminal of operational amplifier 115is connected through diode 120 to the plus input of operationalamplifier 114 and the minus input of 115. Amplifier 115 thus can pulldown the potential at the plus input of operational amplifier 114 whenthis becomes greater than the plus input to operational amplifier 115.The output terminal of operational amplifier 116 is connected to itsminus input terminal and connected through 3.3 kilohm resistor 121 tothe input of operational amplifier 114.

At the point at which amplifier 80 ceases to limit the nozzle referencesignal, the plus and minus inputs of this amplifier are equal. This isrepresented by points such as 122 on FIG. 5 where a line 100 meets line96 and by the point 123 on FIG. 6 where increase of power control inputbegins to increase the speed reference signal. The inputs to operationalamplifier 94 also must be equal to satisfy that amplifier, and currentin resistor 92 must therefore be zero. However, if the power inputsignal increases; that is, the potential on line 20 increases relativeto the inlet temperature signal, this unbalances the input to amplifier94, causing it to conduct and providing the drive current in the basecircuit of transistor 106. The base drive causes current to flow fromthe 10 volt source through resistor 107, diode 108, thecollector-emitter circuit of transistor 106, and resistor 92 to junction90. The magnitude of this current must necessarily be such as to createa drop in resistor 92 equal to the difference in potential betweenjunction 90 and the power input on line 20 to balance the inputs toamplifier 94. Thus, the emitter current of transistor 106 isproportional to the power level input signal or theta. Diode 108 isprovided to balance the emitter-base drop in transistor 110 so that thedrops in resistors 107 and 111 are equal. When transistor 106 conducts,the potential drop across resistor 107 and diode 108 provide a potentialcausing current to flow through resistor 111 and the emitter-basecircuit of transistor 110 to turn on transistor 110. The emitter currentof transistor 110 will therefore be equal to the collector current oftransistor 106 multiplied by the ratio of the resistances, or 10,000divided by 270, or roughly 40 times the current through resistor 92necessary to balance the input of operational amplifier 94. This currentflows through a 1 kilohm resistor 112, which is merely a currentlimiting resistor, and 3.3 kilohm resistor 121 to the output ofoperational amplifier 116. This drives the plus input of operationalamplifier 114 to a value equal to that set on the plus input ofoperational amplifier 116 by potentiometer 119 plus the drop in resistor121. It thus equals the minimum setting plus a value proportional to thesetting of the throttle potentiometer 19. The input follower operationalamplifier 114 provides an equal signal through line 23 to the fuelcontrol. This speed reference signal is compared in the fuel controlwith a gas generator speed signal from the transducer 28 and amplifier32 to regulate fuel flow. This is the normal mode of regulation duringsteady state operation as distinguished from acceleration.

The speed reference signal is limited by the setting of potentiometer118 which prevents the plus input to amplifier 114 from exceeding thisinput by flow through resistor 112 and diode 120 to the output terminalof amplifier 115. This provides a maximum speed reference signal whichis represented by the line 124 in FIG. 6.

Getting back to the turbine nozzle control system, we start with thenozzle control circuit 47 diagrammed at the upper right of FIG. 2. Thenozzle temperature reference signal on line 46 is fed through a 20kilohm resistor 126 to the minus input of an operational amplifier 127.The plus input to this amplifier is the compensated temperature signalfrom the compensating circuit 26 through line 40. The output ofoperational amplifier 127 is connected to its minus input through a 150kilohm resistor 128 and an 11 microfarad capacitor 130 in parallel. Theratio of resistors 128 and 126 results in a 7.5 to 1 gain in theamplifier. The capacitor provides a small amount of lag to stabilize theamplifier. The input from the compensated temperature circuitapproximates actual turbine inlet temperature. As stated above, it isbased upon instantaneous measured turbine inlet temperature pluscompensation for thermocouple lag and modified by certain factors whichcause it to represent more perfectly the anticipated performance of theengine during acceleration.

This provides the feedback to the nozzle reference temperature signal tocause the nozzle to be adjusted to the opening resulting in the desiredturbine inlet temperature at the particular condition of operation.Thus, when the actual temperature signal is fed back against the desiredtemperature signal on line 46, the result is an output indicative of thedesired change in nozzle area which is conducted through a 1 kilohmresistor 131 and 12 kilohm resistor 132 in series to the power amplifierinput line 56. The power amplifier is energized from the plus 12 voltpower bus and provides an output through line 59 to theservo-controlling solenoid 60. The input line to the power amplifier isalso energized off the 12 volt power bus through a 160 kilohm resistor134. The resistor 134 provides an input to the power amplifier such thatthere is a minimum amplifier output to provide a small current to thesolenoid 60 to bias the solenoid and overcome friction or hysteresis innthe valve mechanism of the servo 12.

It will be seen, then, that the amplifier 127 causes the power amplifierto drive the servo so as to open the nozzle if turbine inlet temperatureexceeds that scheduled by the reference temperature signal in line 46and to close the nozzle if turbine inlet temperature is below thatscheduled. This provides for steady-state control of the nozzle area.

Transient Nozzle Control

If there is a sharp increase in the operator's power control signal,there will be a rapid increase in turbine inlet temperature due to theincrease in fuel to accelerate the gas generator. There is also anincrease in the nozzle reference signal due to the increased throttleinput and increasing speed. However, the temperature signal outpaces thereference signal. This simulates an indication that the engine isovertemperature because the nozzle is not open enough, so the naturalresult would be to open the power turbine nozzle. This is advantageousin that it increases the pressure drop across the gas generator turbineand speeds acceleration of the gas generator.

However, the desired acceleration curve of the gas generator may be togive the most acceptable acceleration of the driven vehicle rather thanquickest acceleration of the gas generator turbine. When the nozzle isopened wide, the resulting lack of energy supplied to the power turbinecauses a distinct lag in acceleration of the vehicle. Therefore, oursystem includes means including the nozzle limit logic circuit 51 andnozzle limit circuit 48 to limit the opening of the power turbinenozzle. This includes the nozzle area (or angle) feedback potentiometer50 energized from the 10 volt circuit to ground, the follower of whichis moved by the nozzle actuating mechanism. The voltage taken from thepotentiometer through line 49 represents, and increases with, nozzlearea. This signal is fed to the plus input of an operational amplifier135, acting as a voltage follower with its output connected directly tothe minus input. The resulting signal on the output line 136 is a directfunction of nozzle angle and nozzle area. This signal is fed through 20kilohm resistor 138 to the minus input of an operational amplifier 139,the output of which is connected through 51 kilohm resistor 140 to theminus input. The plus input of operational amplifier 139 is energizedthrough line 52.

For the present, we may assume that the plus input line 52 ofoperational amplifier 139 is supplied with an adjustable fixed potentialfrom a potentiometer 142 energized from the 10 volt supply. The nozzlelimit logic circuits 51 which may vary this potential will be describedbelow.

The ratio of resistors 140 and 138 is such that the gain of operationalamplifier 139 is approximately 21/2 to 1. The output of the operationalamplifier is connected to the junction 143 between resistors 131 and 132through a 620 ohm resistor 144 and a diode 146. Diode 146 allows currentto flow from junction 143 to lower the input to the power amplifier andcause the nozzle to close. When the nozzle area signal fed frompotentiometer 50 through amplifier 135 increases, the potential at theminus input to amplifier 139 increases. If this becomes greater than thepotential on the plus input of amplifier 139, the operational amplifierdraws current through resistor 144 and diode 146 to reduce the input topower amplifier 58 and limit nozzle opening movement at the angle thepotentiometer 142 is set for. Diode 146 prevents the nozzle anglefeedback from overrriding the nozzle refererence temperature signal soas to increase nozzle opening.

As a result of the circuits just described, nozzle angle is limited by afixed maximum and also by a coaction of a temperature limit atemperature feedback. In normal operation, the angle is maintained atthe value to give the temperature called for by the referencetemperature signal on line 46 but, upon acceleration when temperaturemoves up rapidly, the nozzle can open only to the angle set bypotentiometer 142 in connection with the feedback potentiometer 50.

This control can be improved upon, and the means for this is the nozzlelimit logic circuit 51 illustrated in the lower part of FIG. 2 whichprovides means for varying the limit imposed through line 52 on thenozzle opening. We should distinguish between starting conditions; thatis, bringing the engine up to idle speed (50% gas generator speed in thedescribed engine) and normal acceleration; that is, increase of powerafter the engine has reached idle speed and is in the operating range.

The logic circuit 51 provides an override which allows the power turbinenozzle to open to the limit set by potentiometer 142 during cranking ofthe gas generator. This improves starting and reduces the temperature ofthe exhaust gases flowing to the regenerator of the engine if one isprovided. The system also includes means to sense acceleration of theengine above the idle condition, and vary the nozzle area. In a normallyoperating engine, the compensated turbine inlet temperature equals theacceleration reference temperature signal only during engineacceleration. During steady state operation it is limited by the nozzletemperature reference signal and is ordinarily substantially below theacceleration reference signal. Our arrangement for detecting anacceleration condition generates an acceleration signal whenever thecompensated temperature comes within approximately 50° of accelerationreference temperature.

The compensated turbine inlet temperature signal is fed from the circuit26 through line 40 and a 10 kilohm resistor 159 to the plus input of anoperational amplifier 160. The acceleration reference temperature signalfrom the circuit 36 is supplied through line 39 and a voltage divider tothe minus input of amplifier 160. The voltage divider comprises a 10kilohm resistor 162 and a 510 kilohm resistor 163. The potential appliedto the minus input of operational amplifier 160 is thus about 98% of theacceleration reference signal. When the compensated temperature signalis less than 98% of the acceleration reference signal, amplifier 160 isturned off, but when the compensated temperature signal equals orexceeds 98% of the acceleration reference signal, amplifier 160saturates and turns on a transistor 164 (type DS-67). The transistor isturned on by base drive from the amplifier through series resistors 166(1 kilohm), 167 (10 kilohm) and 168 (2 kilohm). Resistor 168 isconnected between the base and emitter of the transistor and grounded at170. This completes a circuit from line 52 through a variable resistor171, the maximum value of which is 50 kilohms, and the transistor toground. Resistor 171 and transistor 164 thus provide a shunt for thegrounded end of potentiometer 142, lowering the potential on line 52 anddriving the nozzle further towards closed position. The amount of nozzleshift can be set by resistor 171.

When the acceleration terminates, the nozzle control reverts to thenormal steady state mode in which the area is controlled by T₄.sbsb.cand the nozzle reference temperature signal through amplifier 127.During the transition from acceleration to steady state, there is ashort period of time while the compensated temperature signal isdropping (as the anticipating features of the temperature compensatedcircuits are dying out) that T₄.sbsb.c would remain greater than thenozzle reference temperature. In order to stabilize the transitionbetween the limit and steady state nozzle control modes, the systemincludes an arrangement for providing a slight delay in the dropout ofthe acceleration override control. This is accomplished by the circuitcomprising diode 172, 22 microfarad capacitor 174, and resistor 175 (20kilohm). When the acceleration condition is sensed and amplifier 160turns on, capacitor 174 is quickly charged through the diode 172. At theend of the acceleration condition the charge on capacitor 174 keepstransistor 164 turned on momentarily until the charge on the condenserdecays through the series circuit of resistors 175 and 168.

During starting; that is, during acceleration from lightoff to idle gasgenerator speed, it is not desired to have the nozzle closed; there isno need for output from the power turbine, and it is desirable to makeas much energy available as possible to the gas generator turbine. It ispossible to use various means, including a speed-responsive switch, tosense the reaching of idle speed by the gas generator. In thisparticular case, the starter is deenergized at about 43% gas generatorspeed, which is near enough for our purposes. Operational amplifier 160is overridden during operation of starter 54 by the circuit to bedescribed. The circuit from 10 volt supply through series resistors 178and 179 (both 10 kilohm) and a diode 180 leads to a point which ismaintained at a low potential during operation of the starter. It may,for example, be a switch closed during operation of the starter or atlow gas generator speed. As illustrated schematically, it is switch 182closed when the starter is energized. Obviously, switch 182 could bespeed-responsive and remain closed below idle speed.

When the switch 182 is closed or the potential at that point isotherwise lowered to a sufficient extent, a transistor 183 (type DS-83)is turned on. This establishes a current flow from plus 10 volts throughresistors 184 (10 kilohm) and 186 (2 kilohm) in series to ground. Thisturns on a second trasistor 187 (DS-67) the collector of which isconnected to the base of transistor 164, the emitter of which isgrounded, and the base of which is connected between resistors 184 and186. This shunts resistor 168 and the emitter-base circuit of transistor164 and turns it off, assuming it would otherwise be on. Thus,regardless of the operation of operational amplifier 160 in response tocorrected turbine inlet temperature and acceleration referencetemperature, the nozzle signal is not held down and the nozzle remainsopen during cranking.

This action may be explained further by reference to FIG. 7, in whichthe line 188 at 3.5 volts represents the open nozzle position signal tothe power amplifier 58 when transistor 164 is turned off. This is thecondition during cranking.

The broken line 190 which starts at 3.5 volts, decreases rapidly to 3.3volts, remains constant until the termination of acceleration, and thenrises to the normal value at point 191, represents the effect of theacceleration circuit embodying operational amplifier 160. Thus, in caseof gas generator acceleration, the operational amplifier 160 provides asignal which causes the nozzle to close and thereby increase the amountof energy available from the power turbine as the vehicle is beingaccelerated.

FIG. 3 illustrates a modification of the circuit of FIG. 2 whichprovides a different characteristic of turbine nozzle area during engineacceleration. The difference involved in FIG. 3 is the insertion of acapacitor 192 (20 microfarad) paralleled by a resistor 193 (300 kilohm)in the circuit between the collector of transistor 164 and the variableresistor 171. In this case, when the compensated temperature signalreaches the value which indicates acceleration of the gas generator, thepotential applied to the power amplifier is reduced as with the circuitof FIG. 2. However, as the 20 microfarad capacitor 192 is charged, thedroop in potential decays exponentially and the potential applied to thepower amplifier rises along the exponential curve 194 in FIG. 7 afterthe initial drop.

The resistor 193 provides for bleed down of capacitor 192 in case ofrapidly succeeding accelerations of the gas generator.

With the characteristic of curve 194, the initial closing of the nozzleas acceleration is initiated provides an increment of energy to thepower turbine. The subsequent opening redistributes the energy availableso that the energy available to accelerate the gas generator isincreased while the portion of the available energy supplied to thepower turbine decreases. However, the total energy supplied to the powerturbine will increase because of the greater temperature and rate offlow through the engine.

If it is desired to have some other characteristic for the line 194 thanthe simple exponential curve illustrated, other types of circuits can beemployed to vary the flow of current through resistor 171 as a functionof time after the beginning of acceleration.

Annotations

We believe that the foregoing detailed description is amply sufficient3,821,562. practice of our claimed invention by those skilled in the artto which it pertains. However, it may be desirable to refer to certainU.S. patent applications, of common ownership with this application,which describe in detail circuits or subsystems which may be employed aspart of the overall engine control diagrammed in FIG. 1. Reference tothe applications may be made if necessary to facilitate understanding ofthe system described and claimed here and the best mode contemplated byus of implementing our invention. The thermocouple amplifier 33 may beas described in Ser. No. 381,055 of Davis, Lopke, and Pechous, filedJuly 20, 1973, U.S. Pat. No. 3,821562. The fuel control 24 may includean amplifier as described in Ser. No. 385,952 of Davis and Lopke, filedAug. 6, 1973, U.S. Pat. No. 3,852,957. The nozzle power amplifier 58 mayembody similar electronic technology. The acceleration referencetemperature signal generating circuit 36 may be as described in Ser. No.343,295 of Oppmann, filed Mar. 21, 1973, U.S. Pat. No. 3,834,158, but weprefer that it be as described in Ser. No. 392,738 of Davis, Lopke, andPechous, filed Aug. 29, 1973, U.S. Pat. No. 3,851,464. The temperaturecompensating circuit 26 may be as described in Ser. No. 392,739 ofDavis, Lopke, and Pechous, filed Aug. 29, 1973.

Turbine motive fluid temperature is measured in our system preferably atthe inlet to the turbine. However, it is known in gas turbine controlsto use instead turbine discharge temperature, which varies ratherclosely with turbine inlet temperature, and this temperature may beregarded as the motive fluid temperature for control purposes. Likewise,it is possible in some cases to use ambient air temperature for engineinlet temperature.

The preferred turbine nozzle structures for gas turbine engines, so faras we are aware, ordinarily involve a nozzle with variable setting anglevanes in which varying the setting changes both the angle of dischargeof motive fluid from the turbine nozzle and the area of the turbinenozzle. There are disclosures of turbine nozzles which have variablearea but not variable angle; and it might conceivably be desirable insome installations to have a nozzle of variable angle but not variablearea. Reference in the appended claims to a variable turbine nozzle isintended to include nozzles in which the angle of discharge, the area ofthe nozzle, or both, are variable.

We believe it will be clear to those skilled in the art from theforegoing detailed description that we have provided improvements incontrols for gas turbine engines, particularly in the control ofvariable turbine nozzles for gascoupled turbine engines; and that thesystem as disclosed herein is particularly well suited to therequirements of practice.

The detailed description of the preferred embodiment of the inventionfor the purpose of explaining the principles thereof is not to beconsidered as limiting or restricting the invention, since manymodifications may be made by the exercise of skill in the art.

We claim:
 1. A system for control of a variable area power turbinenozzle of a gas-coupled gas turbine engine including a gas generator anda power turbine, the system including means responsive to a variabletemperature reference signal and to turbine motive fluid temperaturenormally operative to vary the nozzle area to control turbine motivefluid temperature by opening the nozzle in response to excess of motivefluid temperature over the temperature reference signal; the systemincluding means for generating a signal of a first upper limit to nozzlearea at the initiation of gas generator acceleration in the normaloperating speed range of the gas generator and means for generating asignal of a second upper limit to nozzle area larger than the firstlimit during gas generator acceleration below the normal operating speedrange of the gas generator, the limit signal generating means beingcoupled to the means to vary the nozzle area so as to limit increase inopening of the nozzle in response to the temperature reference signaland the motive fluid temperature.
 2. A system for control of a variablearea power turbine nozzle of a gas-coupled gas turbine engine includinga gas generator and a power turbine, the system including meansresponsive to a variable temperature reference signal and to turbinemotive fluid temperature normally operative to vary the nozzle area tocontrol turbine motive fluid temperature by opening the nozzle inresponse to excess of motive fluid temperature over the temperaturereference signal; the system including means responsive to a turbinemotive fluid acceleration reference temperature signal and to a signalrepresentative of actual turbine motive fluid temperature for generatinga signal of a first upper limit to nozzle area at the initiation of gasgenerator acceleration in the normal operating speed range of the gasgenerator and means for generating a signal of a second upper limit tonozzle area larger than the first limit during gas generatoracceleration below the normal operating speed range of the gasgenerator, the limit signal generating means being coupled to the meansto vary the nozzle area so as to limit increase in opening of the nozzlein response to the temperature reference signal and the motive fluidtemperature.
 3. A system for control of a variable area power turbinenozzle of a gas-coupled gas turbine engine including a gas generator anda power turbine, the system including means responsive to a variabletemperature reference signal and to turbine motive fluid temperaturenormally operative to vary the nozzle area to control turbine motivefluid temperature by opening the nozzle in response to excess of motivefluid temperature over the temperature reference signal; the systemincluding means for generating a signal of a first upper limit to nozzlearea at the initiation of gas generator acceleration in the normaloperating speed range of the gas generator and means for generating asignal of a second upper limit to nozzle area larger than the firstlimit during gas generator acceleration below the normal operating speedrange of the gas generator, the limit signal generating means beingcoupled to the means to vary the nozzle area so as to limit increase inopening of the nozzle in response to the temperature reference signaland the motive fluid temperature; and timing means operative to causethe nozzle area to close to the first limit and then progressively opento the second limit during acceleration in the said normal operatingspeed range.
 4. A system for control of a variable area power turbinenozzle of a gas-coupled gas turbine engine including a gas generator anda power turbine, the system comprising means for generating a nozzlereference temperature signal, means for generating a turbine motivefluid temperature signal, and control means responsive to the saidsignals effective normally to vary the nozzle area so as to cause themotive fluid temperature to follow the reference temperature, the systemincluding means for generating a signal indicative of the nozzle area,means for generating a nozzle area limit signal, and means responsive tothe two last-named signals effective to limit increasing variation ofthe nozzle area by the said control means, the system beingcharacterized by first means for generating the area limit signalincluding settable means providing a first area limit, means fordetecting acceleration of the gas generator effective to override thesaid first means and provide a second area limit smaller than the firstarea limit during such acceleration, and means effective during startingof the gas generator operative to override the acceleration-detectingmeans so as to maintain the first area limit.
 5. A system for control ofa variable area power turbine nozzle of a gas-coupled gas turbine engineincluding a gas generator and a power turbine, the system comprisingmeans for generating a nozzle reference temperature signal, means forgenerating a turbine motive fluid temperature signal, and control meansresponsive to the said signals effective normally to vary the nozzlearea so as to cause the motive fluid temperature to follow the referencetemperature, the system including means for generating a signalindicative of the nozzle area, means for generating a nozzle area limitsignal, and means responsive to the two last-named signals effective tolimit increasing variation of the nozzle area by the said control means,the system being characterized by first means responsive to a turbinemotive fluid acceleration reference temperature signal and to a signalrepresentative of actual turbine motive fluid temperature for generatingthe area limit signal including settable means providing a first arealimit, means for detecting acceleration of the gas generator effectiveto override the said first means and provide a second area limit smallerthan the first area limit during such acceleration, and means effectiveduring starting of the gas generator operative to override theacceleration-detecting means so as to maintain the first limit.
 6. Asystem for control of a variable area power turbine nozzle of agas-coupled gas turbine engine including a gas generator and a powerturbine, the system comprising means for generating a nozzle referencetemperature signal, means for generating a turbine motive fluidtemperature signal, and control means responsive to the said signalseffective normally to vary the nozzle area so as to cause the motivefluid temperature to follow the reference temperature, the systemincluding means for generating a signal indicative of the nozzle area,means for generating a nozzle area limit signal, and means responsive tothe two last-named signals effective to limit increasing variation ofthe nozzle area by the said control means, the system beingcharacterized by first means for generating the area limit signalincluding settable means providing a first area limit, means fordetecting acceleration of the gas generator effective to override thesaid first means and provide a second area limit smaller than the firstarea limit during such acceleration, and means effective during startingof the gas generator operative to override the acceleration-detectingmeans so as to maintain the first area limit; and further characterizedby means for effecting decay of the second area limit toward the firstarea limit after initial establishment of the second limit during suchacceleration.
 7. A system for control of a variable area power turbinenozzle of a gas-coupled gas turbine engine including a gas generator anda power turbine, the system comprising means for generating a nozzlereference temperature signal, means for generating a turbine motivefluid temperature signal, and control means responsive to the saidsignals effective normally to vary the nozzle area so as to cause themotive fluid temperature to follow the reference temperature, the systemincluding means for generating a signal indicative of the nozzle area,means for generating a nozzle area limit signal, and means responsive tothe two last-named signals effective to limit increasing variation ofthe nozzle area by the said control means, the system beingcharacterized by first means responsive to a turbine motive fluidacceleration reference temperature signal and to a signal representativeof actual turbine motive fluid temperature for generating the area limitsignal including settable means providing a first area limit, means fordetecting acceleration of the gas generator effective too override thesaid first means and provide a second area limit smaller than the firstarea limit during such acceleration, and means effective during startingof the gas generator operative to override the acceleration-detectingmeans so as to maintain the first area limit; and further characterizedby means for effecting decay of the second area limit toward the firstarea limit after initial establishment of the second area limit duringsuch acceleration.